Excavation methods and excavators using high voltage electric pulses are previously known. For example, optimization for the crushing of a rock mass and man-made structures by means of electric pulses was described by V F Vajor et al in “Physics Vol. 4” of Tomsk Polytechnic University (Russia) 1996. Another example is by a research group at the Stratchclyde University Scotland UK 2001 where high voltage pulses were used to produce a plasma-channel formation inside the rock ahead of the drill region. The extremely rapid expansion of this plasma channel within the rock, which occurs in less than a millionth of a second, causes the local region of rock to fracture and fragment.
According to this known excavation or drilling method a drill-bit is placed on a rock mass in a discharge liquid. The drill-bit has electrodes integrated into its face. High-voltage pulses are applied to the electrodes at intervals of microseconds to allow electric discharge to pass through the rock mass so as to fracture and crush it. The time required for the rock mass to be fractured is determined by the distance between the electrodes.
Another known version of the method (U.S. Pat. No. 6,164,388) relates to the drilling of holes in the ground and incorporates the feeding of a discharge liquid into the borehole and repeated electric discharges between a plurality of pairs of electrodes which have been arrayed in a suitable arrangement on the face of the drill-bit, said discharges being generated by a stream of high-voltage pulses while at least one of three identified parameters is set at an optimum value for the minimization of the power consumption required for excavation, said parameters being i) the load voltage for the crushing of the matter to be excavated, ii) the single pulse energy and iii) the volume flow of the discharge fluid. Equations are given for the estimation of the optimum values of the parameters and it is substantiated that the optimization significantly influences the efficiency of the drilling energy consumption and progress.
The latter of these known versions of the method describes a related drilling machine consisting of a high-voltage pulse generator placed outside the borehole, a high-voltage into-the-borehole-entry arrangement, a drill-pipe and a drill-pipe guide and a drill-bit mounted at the lower end of the drill-pipe. The drill-pipe incorporates two concentric pipes separated by electric insulators, the inner constituting the high-voltage pipe and the outer the ground pipe, together axially movable within the guide in order to facilitate the drilling progress, said high-voltage pipe being electrically connected to one set of electrodes on the drill-bit and the ground pipe to another, the sets of electrodes together constituting the plurality of electrodes mentioned above. The numbers of electrodes in the two sets are not necessarily equal, but all electrodes are in a fixed arrangement relative to each other, one is in the hole centre, they move axially forward together and the only other movement incorporated is a sector rotational movement of the entire drill-bit around the axis of drilling progress.
The discharge liquid circulating system of this latter drilling machine, the liquid applied normally being diesel- or transformer oil, includes a discharge liquid reservoir, a discharge liquid pump and discharge liquid hoses and pipes. The circulating system allows the discharge liquid to circulate, passing from the reservoir, through the pump and the discharge liquid hoses and pipes to the upper end of the drill-pipe, down through the annulus between the two concentric drill-pipe sections past the insulators as well as inside the high-voltage drill-pipe section, largely freely out under the bit and up the borehole in the annulus between the ground-pipe and the wall of the borehole carrying the excavated cuttings along in the flow, and finally through a flow deflecting nipple at the top of the borehole into hoses and pipes back to the reservoir where the cuttings are separated out before the fluid is re-circulated into the borehole. Out through the bit only the internal high-voltage pipe fluid flow is subjected to directional measures, very limited and with no nozzles incorporated. The annular flow is entirely free and with its much larger cross-section leaves the former totally marginalised.
The reported methods and machines, including the drilling machine described above, which may correctly be labelled “state of the art”, incorporate a number of drawbacks. The borehole external placement of the pulse generator implies the transfer of high-voltage pulses through the entire length of the borehole and the handling of high-voltage at the drill-deck where inflammable substances may occasionally be present, for example during drilling for oil and gas. The machine is thereby potentially controversial from a safety perspective and vulnerable from an insulator breakdown viewpoint for all deeper holes. The concentric twin-pipe concept with its inner annulus dictated by the insulator requirements also infringes on the cross-sectional area of the outer annulus where the cuttings are to pass through thereby increasing pressure requirements, limiting cuttings' size and potentially contributing to the stoppage of flow.
The plurality of electrodes divided in two sets, one high-voltage and one grounded, rigidly arranged relative to each other and only allowed a small sector rotation as a unit around the axis of drilling progress represents another serious drawback from the viewpoint of pulse energy application or, in other terms, pulse energy management:
Assuming a random topography at the bore-front after some drilling has occurred, it appears highly unlikely that any two electrodes will have bottom contact. One will, and whichever for a given pulse turns out to constitute the other half of the pair will, because of the rigid electrode configuration, be separated from the bottom by a smaller or larger liquid-gap thereby forcing the pulse to go off partly in liquid and partly in the bottom matrix thereby obscuring the energy efficiency and slowing down the drilling progress. The only remedy contained in the state of the art for this purpose is the sector rotation allowed, apparently assumed to facilitate a fitting through physical contact between bit and hole-bottom, but qualified judgement indicates that this at best is marginal in effect, probably of no effect at all.
The concept of plurality of electrodes in each set of electrodes contains another drawback. Understandingly it was conceived from the viewpoint of cross-sectional coverage and the reasoning that sooner or later any two electrodes of opposite charge would become the “hot” pair, thereby facilitating overall progress. It overlooked however that another occurrence will be an electrode pair of opposite charge in contact with the hole-bottom, but with such distance between them that the spark will not fly at the given pulse voltage level or it flies in liquid, thereby reducing efficiency and drilling progress.
The consistent placement in the state of the art concept of an electrode, normally a high-voltage electrode in the centre of the borehole constitutes a specific drawback. It means that no pulse discharge will ever occur there. In terms of hole-bottom topography “a mountain-top” will therefore repeatedly develop in the centre of the borehole and uphold drilling progress by the mechanisms mentioned above until it becomes unstable or for random reasons breaks off. There is reason to believe that the drilling speed of the state of the art plasma drilling in reality to a large extent is governed by such a hole-centre scenario.
Cuttings' analysis of the state of the art plasma drilling of dry, hard rock such as granite indicates that very minor physical forces are present in the drilling process, or none at all; no heat, no deformation. This gives reason to assume that the first stage of excavation after the pulse has been applied between to well-placed electrodes is a cutting or a cutting collection placed in a cavity with exact fit as the cutting, the cavity bottom and its surrounds together immediately before constituted the solid hole bottom. A serious drawback in the state of the art electro pulse drilling concept is that there are no or minimal remedies incorporated to cause the cuttings to exit from its indigenous cavity. The free flow of discharge liquid axially from under the bit is the only remedy. Compared to other drilling practices and the hydraulic energy utilized there in order to remove much less dug-in cuttings it would appear totally inadequate. There are therefore reasons to assume that cuttings in state of the art electro discharge drilling remain in place for a substantial time after broken loose and that they receive repeated pulse discharges thereby breaking into smaller pieces before they are finally exited from the bottom of the hole. Lack of efficiency in bottom hole cleaning is widely known from drilling practices in general as a major cause of reduced drilling progress. These practices commonly apply mechanical means to facilitate the cleaning, in addition to the hydraulic; scraping, cutting and hammering.
The annular hydraulic lifting of cuttings requires circulating fluid velocities and viscosities that have been substantiated through many generations of drilling practise. For large cuttings and dry hard rock of high density such as granite, the requirements are at their maximum. The use of pure transformer or diesel oil as a discharge fluid puts the state of the art electro discharge drilling technology at a significant distance from these requirements. In order to conform, the viscosity must be increased and the flow regime maintained at higher pressure differentials than currently used. Likelihood is that the state of the art technology after repeated cuttings breakage moves the cuttings to the periphery of the bit from where it sets up a temporary flow-loop a short distance up the annulus until a slug has been built up at which time it travels up and emerges in the form of slug flow. This is another facet of inadequate bottom hole cleaning which constitutes a serious drawback by slowing the drilling speed.
In GB patent specification (Tylko 1966) arc plasma is used as a heat source and the circulation liquid has a quenching function in addition to the removal of residues, i.e. the cuttings, of the drilling. Arc plasma drilling has never been successful in high speed operations.